Thermoplastic polymers used in the internal components of hard disk drives should be capable of varying operating temperatures. In particular, the thermoplastic components should capable at operating temperatures of above 150° C. Also, moving parts within the hard disk drive, for example, latches, require a self-lubricating material with a reduced coefficient of friction. Blends of a thermoplastic polymer such as polyetherimide and polytetrafluoroethylene are capable at varying operating temperatures. Additionally, blends of polyetherimide and polytetrafluoroethylene have a low friction factor. There are, however, disadvantages associated with the polymer blend.
Polytetrafluoroethylene (PTFE) is a typical additive for thermoplastics. It is used to provide materials with good wear performance and a low coefficient of friction. Polytetrafluoroethylene micro powders are typically generated through direct polymerization. Directly polymerized PTFE has a strong tendency to fibrillate under shearing at room temperature because crystallization is initiated above the glass transition temperature of 17° C. Typically, directly polymerized PTFE is treated with radiation to lower the molecular weight and improve handling. Typical leachable fluoride levels measured by Ion Chromatography (IC) for directly polymerized PTFE are several ug/g while the levels for irradiated PTFE might be several hundred ug/g. The fluoride ion level will be even higher after compounding or injection molding, where process temperatures may exceed 340° C., the point at which polytetrafluoroethylene begins to degrade.
The typical compounding and molding process temperatures for polyetherimide may be as high as 370° C., which may present a problem for polytetrafluoroethylene, because as previously mentioned, it begins to degrade at temperatures above 340° C. Due to this degradation, the fluoride ion level of the final product after processing could be up to several hundred ug/g more than normal. Fluoride ions are a main concern for compounds used in the semiconductor industry. Industry specifications for fluoride ions, as tested by leachable ionic chromatography, require the levels be no higher than 40 ug/cm2, or about 40 ug/g. As a result, the fluoride levels for irradiated polytetrafluoroethylene do not meet the required specifications.
A typical method utilized to reduce the fluoride levels of molded parts is to perform a rinse with deionized water, after molding. However, ultrasonic water washing is an additional process used to clean the part after molding in order to reduce the ionic level on the surface. This additional step may add to the process cost and increase the leadtime to molders. This process is not suitable for parts with metal inserts, as the fluoride ion will attack the metal surface. Another method to control the level of fluoride generated during compounding is to reduce the process temperature, preferably below 340° C. Unfortunately, even with the reduced process temperature, there is still the risk of the polytetrafluoroethylene being exposed to temperatures higher than 340° C. during the downstream process, for example during molding. As a result, the polytetrafluoroethylene will still generate higher levels of fluoride ions.
Another approach to reduce fluoride levels is to use non-irradiated, directly polymerized polytetrafluoroethylene for compounding. This non-irradiated version of polytetrafluoroethylene is effective in controlling the fluoride level to several ug/g, which is lower than irradiated polytetrafluoroethylene. However, although the fluoride levels are lower with non-irradiated polytetrafluoroethylene, the cost of handling non-irradiated polytetrafluoroethylene micro powder is high.
Accordingly, there is a need to reduce the fluoride level of compounds containing irradiated polytetrafluoroethylene. These compounds are beneficially able to withstand the compounding or molding process temperatures as well as a downstream deionized water or water based solvent washing process. At the same time, it would be beneficial for the compounds to maintain leachable fluoride levels lower than the industry specification. In addition, it would be beneficial to provide a thermoplastic composition suitable for use in the internal components of hard disk drives, as well as medical or food or beverage applications requiring controllable fluoride levels.